Visible crew/equipment: When they are taping the family, Carol gets mad and goes upstairs and the camera guy taping her tapes her going up the stairs and when he does, for a second, you can see where the set ends and all the lights.
Revealing mistake: When the band is playing the show at the end, JT is supposed to be playing the bass, but anyone can tell he is not, even when the bass sound is used.
Continuity mistake: In this episode, Steve Urkel appears, depicted as a real person. But Urkel also appeared in a few episodes of Full House, which, within the universe of Step by Step, is a fictional television series.
Answer: I could be mistaken, but when the series debuted, its premise was something along the lines of "A divorced contractor and a widowed beautician impulsively marry." If that is true, then the Foster children's father is dead. As for Frank, it could be that his wife was an unfit mother and is therefore not in her children's lives. Perhaps the writers felt addressing these issues would lead to more questions, or at the least require characters to bring up the unseen parents more in conversation, so perhaps they felt it was just better to never talk about them at all and let any discussions about them remain off-screen.
Phaneron ★